[In vitro and in vivo diagnosis of borderline hyperthyroidism]

The evaluation of "preclinical hyperthyroidism" includes laboratory and in vivo tests. The measurements of circulating total thyroxine (T4) and total triiodothyronine (T3) depend on the plasma concentration of thyroid hormone binding proteins. A simple approach, the calculation of the free T3 index (FT3I) and the augmented FT3I (aFT3I) from total T3 and T3-resin uptake corrects the total serum T3 for variation in thyroid hormone binding capacity. In a group of 95 patients with border-line elevated T3 levels we have correlated the results of FT3I and aFT3I with clinical, biochemical and radioisotope findings. It was shown that in patients with moderate elevation of T3 the FT3I and aFT3I are capable of distinguishing clearly between the euthyroid and hyperthyroid range. To detect an imbalance in the hypothalamic-pituitary-thyroid axis either the T4-suppression test or the TRH-test could be used. In 60 patients with "preclinical hyperthyroidism" we have compared both of them. In 77% of the patients the T4-suppression test and i.v. TRH-test provided concomitant results. In 13 patients without response to the i.v. TRH-test 7 showed a definite (more than 50%) and 6 a partial suppression (10-40%) of radioiodine uptake. This suggests that there is no sharp distinction between hyper- and euthyroidism.